1. Field of the Invention
The present invention relates to a process for forming a full-color image by an electrophotographic system or an electrostatic printing system, and more particularly to a process for forming a full-color image making use of a black toner, a cyan toner, a magenta toner and a yellow toner, by which both low-temperature fixing ability and hot offset resistance are made excellent.
2. Description of the Related Art
In image forming apparatus of an electrophotographic system (including an electrostatic printing system), such as copying machines and printers using toners for development of electrostatic images, the speeding-up of printing, lowering of a fixing temperature and formation of full-color images are strongly required.
When full-color printing is conducted by using an image forming apparatus of the electrophotographic system, color spectra visible to the naked eyes are reproduced by using the principle of subtractive color mixture of three basic colors of yellow, cyan and magenta. Specifically, in the full-color printing, a yellow toner, a cyan toner and a magenta toner are used to form a full-color image. In addition to these 3 toners for full-color images, a black toner is also generally used.
The toner of each color comprises colored resin particles containing a binder resin and a colorant. Various kinds of additive components such as a charge control agent, a parting agent and a dispersing agent are contained in the colored resin particles as needed. As the colorant, is used a yellow colorant, a magenta colorant, a cyan colorant or the like. As a colorant for the black toner, is used a black colorant such as carbon black.
Toners are roughly divided into a pulverized toner by a pulverization process and a polymerized toner by a polymerization process. Examples of the polymerization process include an emulsion polymerization process, an aggregation process, a dispersion polymerization process and a suspension polymerization process. According to the polymerization process, toner particles of micron order can be directly obtained in a relatively narrow particle diameter distribution. According to the suspension polymerization process among these polymerization processes, a polymerized toner having a spherical form, a sharp particle diameter distribution and a small particle diameter can be obtained.
When a polymerizable monomer for shell is polymerized in the presence of the colored polymer particles formed by the polymerization process to cover the colored polymer particles with a polymer layer, a core-shell type toner (also referred to as “capsule toner”) can be obtained. When the binder resin of the colored polymer particles, which will become core particles is formed by a polymer having a relatively low glass transition temperature, and the polymer layer, which will become a shell, is formed by a polymer having a relatively high glass transition temperature, a polymerized toner well balanced between low-temperature fixing ability and storage stability (blocking resistance) can be obtained.
As described above, the polymerized toner is excellent in image reproducibility because the particle diameter can be made small, and moreover permits low-temperature fixing and can be made hard to cause hot offset by providing the toner as the core-shell type.
When a color image is formed with the spherical toners of the core-shell type, however, cleaning using a cleaning blade is made difficult. In addition, the formation of the color image tends to more cause hot offset than the case of a monochromatic image because the color image is formed by superimposing 4 color toners on one another.
In the core-shell type toner by the polymerization process, it has heretofore been adopted to reduce the amount of a remaining monomer and the amount of a remaining organic compound, or add a parting agent, which can make the resulting toner hard to cause hot offset, into the toner.
U.S. Pat. No. 6,887,637 B2 (hereinafter referred to as “Article 1”) has proposed a toner for development of electrostatic images containing at least a binder resin, a colorant and a charge control agent, in which a copolymer containing a quaternary ammonium salt group-containing (meth)acrylate monomer unit and having a glass transition temperature of 40 to 75° C. is used as the charge control agent. Article 1 also shows that the toner is provided as the core-shell type.
U.S. Patent Application Publication No. 2005/0250033 A1 (hereinafter referred to as “Article 2”) shows that a magenta toner containing C.I. Pigment Red 31 and 150 in combination as magenta pigments is excellent in low-temperature fixing ability and hot offset resistance.
U.S. Patent Application Publication No. 2007/0172751 A1 (hereinafter referred to as “Article 3”) discloses a toner for development of electrostatic images made up of colored resin particles containing a binder resin, a colorant and a parting agent, in which (1) the volume average particle diameter of the colored resin particles is 4 to 9 μm, (2) the average circularity of the colored resin particles is 0.93 to 0.995, (3) a shear viscosity at a temperature of 130° C. and a shearing rate of 10/sec is 3,500 to 8,000 Pa·s, (4) a shear viscosity at a temperature of 130° C. and a shearing rate of 500/sec is 300 to 1,300 Pa·s, (5) a content A of a component having a volatilizing temperature of 130° C. or lower is 100 ppm or lower, (6) a content B of a component having a volatilizing temperature higher than 130° C., but not higher than 180° C. is 100 ppm or lower, (7) A+B is 150 ppm or lower, and (8) A/B is 1.0 or smaller.
U.S. Patent Application Publication No. 2006/0269866 A1 (hereinafter referred to as “Article 4”) discloses a process for producing a polymerized toner excellent in durability upon printing by containing a water-soluble polymerization inhibitor and thiuram disulfide in a dispersion liquid of a polymerizable monomer composition.